Techniques for the measurement of CH4 oxidation in landfill cover soils and other ecosystems were recently reviewed by Nozhevnikova et al. (2003).
Boeckx et al. (1996) determined CH4 oxidation in a cover soil by removing the cover on part of the landfill and measuring the CH4 flux with a closed-box method directly on the waste as well as on the surrounding cover soil. Similar techniques have been used in wetlands (Nozhevnikova et al., 2003). The advantage of this method is that it is a direct measurement. The disadvantage is the amount of disturbance required to perform the measurement, creating a bypass for the gas.
Czepiel et al. (1996a) developed a method to estimate the year-round, whole-landfill average CH4 oxidation by combining field, laboratory and computer modelling methods. They used CH4 concentrations at 7.5 cm depth as a proxy for the soil CH4 oxidizing capacity, and combined this information with a typical depth profile of the activity as well as representative corrections for temperature and soil moisture contents. A year-round whole-landfill average CH4 oxidation for a landfill in New Hampshire was 10%. The method is non-intrusive, but it is unclear if the proxy and the corrections are representative of other landfills in other climates.
Kjeldsen et al. (1997) estimated the CH4 oxidizing capacity of a landfill cover soil by integrating the depth profile of the CH4 oxidation rate of soil samples taken at different depths in the cover, as determined in the laboratory.
Oonk and Boom (1995) used a mass balance method to determine CH4 oxidation in cover soils of several landfills in the Netherlands. They measured CH4 and CO2 fluxes leaving the landfill cover soil and compared the CH4/CO2 flux ratio with the CH4/CO2 production ratio inside the waste. This enabled them to deduce the CH4 oxidation efficiency. The main advantage of this method is that it is direct and non-intrusive. The disadvantage is the potential interference of CO2 production and consumption by plants that is sometimes more important than the landfill CO2 production. Oonk and Boom (1995) solved this problem by taking the average of a 24 h monitoring.
A very promising method to determine CH4 oxidation in landfill cover soils is by stable isotope measurements. Methanotrophs oxidize 12CH4 slightly faster than 13CH4 (Barker and Fritz, 1981; King et al., 1989). A similar effect was observed for CH3D (Coleman et al., 1981). This effect can be used to quantify CH4 oxidation by measuring the 813C or 8D abundance in CH4 emitted at the cover soil surface and comparing them with the abundance in CH4 produced inside the landfill (Liptay et al., 1998; Chanton and Liptay, 2000; Borjesson et al., 2001). A simple equation to relate changes in the isotope abundance to the fate of a compound was provided by Blair et al. (1985):
where 8E is the 813C abundance of the emitted CH4 (%o), and 8A the 813C abundance of the produced CH4. aox and otrans are frac-tionation factors of oxidation and transport, respectively. Usually, atrans is assumed to be equal to 1. However, recent research has indicated that gas transport also causes isotope fractionation (atrans> 1), which leads to an underestimation of CH4 oxidation by the isotope method (De Visscher et al., 2004). Scharff et al. (2003) compared the mass balance method with the isotope method. They found that the results were not significantly different, but the uncertainty of the methods was very large.
More qualitative methods to test the occurrence of CH4 oxidation include meth-anotrophic cell counts in the cover soil and the measurement of the N2/O2 ratio in the landfill cover soil (Nozhevnikova et al., 1993, 2003).
Determination of in situ CH4 oxidation can also be carried out using selective inhibitors for CH4 oxidation. Krüger et al. (2001) tested the selectivity for difluoromethane (CH2F2) and showed that 1% CH2F2 had no effect on methanogenesis. The difference between CH4 emission found with and without the use of CH2F2 is then a measure for the in situ CH4 oxidation rate.
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